As we learnt earlier coal is one of the most widely used fuels in thermal power plants. It is therefore necessary for anyone interested in thermal power plants to know about the composition of coal and its various ingredients which have been discussed in the succeeding section.
Proximate Analysis of Coal
Any substance can be analyzed in different ways such as proximate analysis, chemical analysis and so forth. We will carry out the proximate analysis of coal which gives the different categories of compound present in the substance. Apart from carbon which is an obvious constituent the other constituents are as follows.
Ash – this is an undesirable constituent of coal which is contained within the coal in two forms namely fixed ash and free ash. Fixed ash is inherent in the coal due to the formation process from vegetable matter and it is not possible to remove it except that it gets separated on burning. The free ash is removable via processes such as washing and screening though they will not be described in detail here.
Like I said earlier it is undesirable to have ash in the coal but normally different types of coal could have ash content anywhere between say around 2% to 30% which is an unnecessary burden adding to transportation costs, lowering of heat value of coal and producing large amounts of corrosive waste which needs to be disposed off in a proper manner. Another disadvantage is that inside the boiler combustion chamber if the ash gets subject to very high temperatures it can form clinkers which could choke the passages and decrease efficiency of the boiler. It is best to use such coal in the powdered form if such a possibility exists.
Hydrocarbons & Gases – coal contains a variety of combustible gases such as hydrogen, methane and non-combustible gases such as carbon dioxide etc. This volatile content could be as low as 3% or even as high as nearly 50%. These constituents especially the non-combustible gases are just a waste as far as heat value of the fuel is concerned and an unnecessary burden which needs to be transported around and stored without any useful value in return.
Moisture – the moisture content of coal could vary from just over 1% to nearly 30-40% and just like ash it has two forms – inherent moisture and free moisture. The former is not easily removable as it is a costly process whilst the latter can be removed by normal drying using slightly heated air. Obviously since coal is used in the boiler combustion chamber any excess moisture would interfere with the combustion lowering the actual heat available to generate steam.
There is another method of analyzing coal in terms of its various chemical constituents such as carbon, hydrogen, sulphur and so forth but that might be taken up in a different set of articles.